This pragmatic adaptive clinical trial will test the effects of a Stigma-Motivational-Decision intervention designed to increase HIV treatment engagement, retention, and medication adherence for substance using adolescents and young adults (AYA) living with HIV who are not in clinical care. The intervention uses a uniquely unified counseling approach at multiple points along the HIV continuum of care. In a first step, we will use multiple modes of outreach including social media, passive media, clinic records, and chain referrals to seek and identify HIV positive AYA (age 16 to 25) who are out of HIV care. In Step 2, HIV positive AYA who are out of HIV care and substance using will be enrolled in a phone-delivered counseling intervention to address substance use, HIV stigmas, medical care-related concerns, structural barriers, and other challenges to engaging youth in HIV care. Counseling will be provided weekly until the time participants are engaged in care, with a maximum of 12 sessions (up 3-months). We will therefore determine the number of theory-based phone intervention sessions needed (minimally effective dose) for care engagement and the associated costs. Once engaged in care, Step 3 will conduct a randomized trial to test the comparative effects and cost effectiveness of interactive text messaging with opportunities for ongoing supportive problem solving versus text message reminders to sustain long-term retention in care and medication adherence. HIV positive AYA are likely to drop out of care because of known challenges such as substance use, social barriers such as stigma, and concerns regarding treatment. Our 3-step study will use a single theory-based approach to determine the number of phone-delivered Stigma-Motivational-Decision counseling intervention sessions necessary to achieve engagement or re-engagement in HIV care, and will test the effects of a low-cost approach to sustaining long-term retention in care and medication adherence. Participants will be out of care at baseline and counseled with up to 12 weekly phone-delivered intervention sessions until they confirm engagement in HIV care. The study will determine the minimally effective counseling dose needed to engage substance using HIV positive AYA in care. Once participants attend HIV medical appointments, they will be randomized to either: (a) 15-months of ongoing weekly mobile phone text message check-ins with the opportunity for brief problem-focused phone support or (b) weekly text message reminders. The study will test the effects of the counseling and text messaging interventions on maintaining HIV care retention, medication adherence, and HIV viral suppression over 18-months. The primary endpoint in this trial is HIV RNA suppression. We will perform economic evaluations to determine the cost-effectiveness of the engagement-retention-adherence intervention. The proposed pragmatic adaptive trial will therefore determine the minimally effective dose of counseling and the effects of a low-burden retention/adherence intervention to maintain retention in care and avoid relapse to non-adherence for AYA living with HIV.